This invention relates to passive vibration isolation and more particularly to a means of reducing the vibrations transmitted to spacecraft from their launch vehicles during the launch process. The benefits include reduced structural weight and cost, as well as increased life and reliability, of the spacecraft and its components.
Historically, the connection between the spacecraft and the launch vehicle has been made with a very stiff spacecraft adapter. That is generally considered to be a "hard mount" and is extremely efficient at transmitting all structure-borne forces from the launch vehicle to the spacecraft over a very wide frequency band. A need exists for isolating the payload of a launch vehicle from structure-borne vibrations due to launch, maneuvering, thrust termination and staging, as well as periodic thrust oscillations, pyrotechnic separation systems and aerodynamic loading.
Vibration isolation systems work by connecting the isolated structure (payload) to the base structure by means of a resilient mount or mounts. All isolation systems are mounting systems. If the isolated payload is also mounted by other than isolation elements, then those elements would "short circuit" the isolation system, and thereby prevent isolation. The payload has certain mass properties (i.e. inertia) which tend to make it stand still in inertial space even during vibration of the base structure from which it is supported by the resilient mounts. The degree to which the payload vibration is less than that of the supporting or base structure (i.e., the level of vibration isolation) is determined by the properties of the resilient mounts, by the geometry of their mounting to the payload and base structure, and by the mass properties of the payload. The resilient mounts must have low relative stiffness as compared to the base and payload, and they also must have some degree of structural damping. The stiffness of the resilient mounts is tuned so that the frequency of vibration of the supported payload on the resilient mounts is a specified value, i.e. the isolation frequency, which results in the payload being effectively isolated from dynamic loads of frequencies higher than the isolation frequency. Damping is required in the resilient mounts so to reduce the amplitude of response of the payload at the isolation frequency when the system is under external excitation at the isolation frequency. The resilient mounts must also allow sufficient relative motion between the vibrating base structure and the payload, which is referred to as the "rattle space."
Because the spacecraft is a major structural component of the launch vehicle/spacecraft dynamic system, variations in the isolation frequencies greatly effect the dynamics of the launch vehicle/spacecraft system. Any unpredicted changes in the dynamics can have an adverse effect on the control system of the launch vehicle and cause instability and thereby loss of the mission. Therefore, the stiffness properties of the isolation system must be well predicted and accounted for for the duration of the flight. The simplest and most effective way of achieving this predictable isolation system performance is by having a completely linear isolation system under all load cases, including launch vehicle acceleration loads from -2g's to +6g's. Resilient mounts commonly use a soft, non-linear material, such as an elastomeric, as their stiffness component. However, because of their non-linearity, elastomerics (rubbers, etc.) exhibit different stiffness under various loads, temperatures, and frequencies, resulting in complexity and unpredictability in performance, and therefore they cannot effectively be used as the stiffness component of a Whole Spacecraft Vibration Isolation System. Also, under very high static loads, elastomerics creep (deflect as a function of time), and this cannot be tolerated. The use of elastomeric material as the stiffness component has been due to its heretofore advantage in tolerating strains up to 50%, which has allowed the elastomeric isolation mount to provide the necessary rattle space.
Owners of spacecraft that cost tens to hundreds of millions of dollars demand a high strength, high fatigue-life connection between the spacecraft and the launch vehicle. This connection must provide a fail-safe connection; must be able to handle, without stress failure, the deflections due to the sum of the quasi-static acceleration loads of the spacecraft due to maneuvering and other vehicle loading events, and the dynamic loads of the isolation system; must be completely linear in all deflection regions (both tension and compression); and must be of minimal height (reducing payload volume) and minimal weight (reducing overall payload weight). The isolation system must also not introduce collateral problems with the launch, such as low frequency modes, interaction with the launch vehicle control system, or reduction of payload-fairing clearance. The isolation system must be easily tunable for different combinations of launch vehicles and spacecraft, and readily employable in existing spacecraft because flight heritage is important.
Whole-Spacecraft vibration isolation design has eluded previous attempts. The disclosed invention, which is elegant and simple, satisfies all of these requirements and has been proven with actual flight usage.